The present invention relates to a pretransfer charging device for an electrophotographic or electrostatic copier, facsimile transceiver, printer or similar image forming equipment.
Predominant type of electrophotographic image forming equipment, for example, applies a charge or a bias voltage to an image transferring medium so as to electrostatically transfer a toner from an image carrier to the transferring medium. In this kind of image transfer system, the amount of charge deposited on the toner contributes a great deal to the image transferring ability since the toner is driven by an electric field developed between the image carrier and the transferring medium. When the amount of charge deposited on the toner is greater or smaller than desired for optimal for image transfer, the following drawbacks (1) and (2) are brought about.
(1) When the actual amount of charge is greater than the desired one, the electrostatic force acting between the toner and the image carrier increases to obstruct the transfer of the toner from the image carrier to the transferring medium despite the electric field.
(2) When the actual amount of charge is smaller than the desired one, the above-mentioned electrostatic force decreases to cause the toner to be transferred even by a weak electric field. Specifically, even when the transferring medium is not held in sufficient contact with the image carrier, the toner is transferred from the image carrier toward the transferring medium. This degrades the quality of a reproduction due to the scattering of toner. Moreover, the inside of a character or similar line image and the edge of a solid image having a substantial area are often lost on a reproduction when the amount of charge is small, although why this occurs has not been accounted for yet.
In light of this, it is a common practice to use a pretransfer charger which deposits a charge on the toner image formed on the image carrier before the transfer, so that an optimal charge for image transfer may be deposited on the toner to be transferred. The pretransfer charger is an extremely effective implementation against the above-described drawbacks since the amount of charge to be deposited on the toner can be controlled by a charge fed from the outside.
However, experiments have showed that when the output of the pretransfer charger is constant, the amount of charge to be deposited on the toner cannot be controlled to a constant value. Specifically, when use is made of a certain kind of developer, the charging characteristic of the developer noticeably changes due to the ambient conditions or aging to in turn change the amount of charge deposited on the toner.